All cancers are caused by a change in DNA, but most cancers are not hereditary. If genetic mutations occur that damage the way cells regulate their growth and death, cancer can be the result. Oncogenes are mutated genes that convert normal cells into cancerous cells. Before they are mutated, oncogenes are called proto-oncogenes. Proto-oncogenes are normal genes that carry the information that tells a cell when to divide. When a proto-oncogene mutates into an oncogene, cells can grow out of control.

Other genes in the human genome, called tumor suppressor genes or anti-oncogenes, are responsible for helping prevent cancer. The protein that these genes make inhibits mitosis. When a tumor suppressor gene mutates, it loses its ability to do this. However, a mutated tumor suppressor gene acts as a recessive gene. In other words, both tumor suppressor gene alleles on two homologous chromosomes need to be damaged before the tumor suppressor gene loses its function. Oncogenes, on the other hand, act as dominant genes. If just one proto-oncogene is damaged, cells can grow out of control. Mutations in proto-oncogenes or tumor suppressor genes can be caused by many things—for example, exposure to a carcinogen such as cigarette smoke or excessive sunlight (radiation). Genetic mistakes that occur in the copying of DNA during cell division can also occur. Most of the time, these mistakes are corrected, but sometimes they are not, and then the mutation is passed down to daughter cells when a cell divides. Over time, as we get older, these mistakes build up, which is the reason why more people get cancer as they age.

Sometimes mutations in tumor suppressor genes can be inherited. But just because someone inherits a mutation does not necessarily mean they will develop cancer. It usually takes several mutations to cause cancer. However, inheriting an existing mutation does raise the risk. This increased risk is called a genetic predisposition to developing cancer. For example, women who inherit the mutated BRCA1 or BRCA2 gene are more likely to get breast cancer than women who inherit normal BRCA1 or BRCA2 genes. However, only 5% of women who develop breast cancer have inherited a damaged BRCA1 or BRCA2 gene. The rest of breast cancer cases are not caused by an inherited risk, but by exposure to carcinogens or just by random chance.

A cancer of the eye called retinoblastoma can form if a fetus inherits a chromosome 13 from a parent who has a deleted RB (retinoblastoma) allele. The RB gene is a tumor suppressor gene, so it inhibits mitosis. In order for the baby to develop retinoblastoma, however, another random mutation must occur in another eye cell that damages the other RB allele. So it is possible to inherit a damaged gene that might predispose someone to developing cancer, but this does not necessarily mean that the person will develop cancer.